absorbent articles

Abstract

To provide an absorbent article for maintaining a water absorption magnification and a water absorption speed, absorbing body fluid in a wide range of viscosity from body fluid with low viscosity to body fluid with high viscosity, and reducing a return amount when absorbing the body fluid.SOLUTION: There is provided an absorbent article comprising: a top sheet which is a liquid permeable top sheet and whose basis weight amount is 25 g / m2; an absorber including a polyacrylic acid(polyacrylate) based water absorptive resin in which a free swelling speed A in a polyethylene oxide solution at 40°C is 0.15 g g-1 s-1 or greater, a free swelling speed B in physiologic saline at 40°C is 0.40 g g-1 s-1 or greater, a ratio of the free swelling speed (A) / free swelling speed (B) is equal to or greater than 0.20; and a liquid impermeable back sheet in the order. In the absorbent article, a basis weight of the water absorptive resin is 100-1000 g / m2.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an absorbent article, and more specifically to an absorbent article having an absorbent body containing a polyacrylic acid (salt)-based water-absorbent resin. [Background technology]

[0002] Known absorbent articles include sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads that absorb body fluids. In recent years, water-absorbent resins have been widely used as water-absorbing agents in these sanitary materials, as constituent materials thereof, from the viewpoint of absorbing body fluids. Known examples of such water-absorbent resins include hydrolyzates of starch-acrylonitrile graft copolymers, neutralized products of starch-acrylic acid graft polymers, saponified products of vinyl acetate-acrylic acid ester copolymers, and crosslinked products of partially neutralized (meth)acrylic acid polymers. However, from the viewpoint of water-absorbing performance, poly(meth)acrylic acid (salt)-based water-absorbent resins using (meth)acrylic acid and / or its salts as monomers are most widely produced industrially.

[0003] The performance required for water-absorbent resins used in sanitary materials, from the viewpoint of body fluid absorption, includes absorbency and absorption rate. These are parameters of absorption characteristics that are generally measured using a test liquid simulating urine (for example, a 0.9% by mass sodium chloride aqueous solution) (Non-Patent Document 1). Meanwhile, water-absorbent resins suitable for absorbing body fluids with higher viscosity than urine, such as blood and loose stool, have also been studied (Patent Documents 1 to 3).

[0004] Furthermore, absorbent articles using a core wrap of highly liquid-permeable paper for absorbing highly viscous liquids have also been investigated (Patent Document 4). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2020 / 137241 Brochure [Patent Document 2] International Publication No. 2002 / 085959 Pamphlet [Patent Document 3] International Publication No. 2011 / 023572 Pamphlet [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-148060 [Non-patent literature]

[0006] [Non-Patent Document 1] Modern Superabsorbent Polymer Technology (published in 1998 by Fredric L. Buchholz and Andrew T. Graham Publisher: WILEY-VCH) Summary of the Invention [Problem to be solved by the invention]

[0007] The viscosity of body fluids to be absorbed in sanitary materials (particularly disposable diapers) is not constant when, for example, urine is excreted together with loose stool. Conventional water-absorbent resins have not been able to achieve both water-absorbing performance for low-viscosity body fluids (e.g., urine) and water-absorbing performance for high-viscosity body fluids (e.g., a mixture of urine and loose stool). Furthermore, merely improving components other than the water-absorbent resin, as disclosed in Patent Document 4, was insufficient to improve the absorption rate of high-viscosity body fluids.

[0008] The problem to be solved by the present invention is to provide an absorbent article that can absorb body fluids with a wide range of viscosities, from low-viscosity body fluids (e.g., urine) to high-viscosity body fluids (e.g., a mixture of urine and loose stools), while maintaining a water absorption capacity and a water absorption rate around human body temperature, and that has a small amount of rewet when absorbing the body fluids. [Means for solving the problem]

[0009] An absorbent article according to one embodiment of the present invention is an absorbent article comprising, in this order, a liquid-permeable top sheet, an absorbent body containing a water-absorbent resin, and a liquid-impermeable back sheet, wherein the basis weight of the water-absorbent resin is 100 to 1000 g / m2 and the top sheet has a basis weight of 25 g / m 2 The absorbent article is as described above, wherein the water-absorbent resin is a polyacrylic acid (salt)-based water-absorbent resin that satisfies the following requirements (a) to (c): (a) The free swelling rate (A) in an aqueous solution of polyethylene oxide at 40°C is 0.15 g g -1 ·s -1 That's it; (b) The free swelling rate (B) in saline at 40°C is 0.40 g g -1 ·s -1 That's it; (c) The ratio of free swelling rate (A) to free swelling rate (B) is 0.20 or more.

[0010] In the absorbent article according to one embodiment of the present invention, the water-absorbent resin has a CRC of 25 to 50 g / g.

[0011] In an absorbent article according to one embodiment of the present invention, the water-absorbent resin is an aggregate of spherical particles.

[0012] In an absorbent article according to one embodiment of the present invention, the bulk density of the water-absorbent resin is 0.40 to 0.80 g / cm 3 is.

[0013] In an absorbent article according to one embodiment of the present invention, the water-absorbent resin has a free swelling rate (A) of 0.25 g g in a polyethylene oxide aqueous solution at 40°C. -1 ·s -1 That's all.

[0014] An absorbent article according to one embodiment of the present invention is an absorbent body in which the absorbent body further contains hydrophilic fibers, and the mass of the water-absorbent resin is 50% by mass or more and less than 100% by mass of the total mass of the water-absorbent resin and the hydrophilic fiber material. [Effects of the Invention]

[0015] According to one aspect of the present invention, it is possible to provide an absorbent article that has an excellent absorption rate not only for low-viscosity body fluids (e.g., urine) but also for high-viscosity body fluids (e.g., a mixture of urine and loose stools), while maintaining the water absorption capacity of the water-absorbent resin and the water absorption rate at around human body temperature, and that has a small amount of return when absorbing the above-mentioned body fluids. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention will now be described with reference to the best mode thereof. It should be understood that the terms used in this specification are used in the same manner as commonly used in the art unless otherwise specified. Therefore, unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the art to which this invention belongs. In the event of any conflict, the present specification (including definitions) shall prevail. The present invention is not limited to the following embodiments, and various modifications may be made within the scope of the claims.

[0017] An absorbent article according to one embodiment of the present invention is an absorbent article comprising, in this order, a liquid-permeable top sheet, an absorbent body containing a water-absorbent resin, and a liquid-impermeable back sheet, wherein the basis weight of the water-absorbent resin is 100 to 1000 g / m 2 and the top sheet has a basis weight of 25 g / m 2 The absorbent article is as described above, wherein the water-absorbent resin is a polyacrylic acid (salt)-based water-absorbent resin that satisfies the following requirements (a) to (c): (a) The free swelling rate (A) in an aqueous solution of polyethylene oxide at 40°C is 0.15 g g -1 ·s -1 That's it; (b) The free swelling rate (B) in saline at 40°C is 0.40 g g -1 ·s -1 That's it; (c) The ratio of free swelling rate (A) to free swelling rate (B) is 0.20 or more.

[0018] The water-absorbent resin used has a high swelling rate for both low-viscosity and high-viscosity liquids. By using such a water-absorbent resin, even if the water-absorbent resin has a high basis weight, the entire water-absorbent resin contributes to absorbing the liquid, leading to a reduction in the amount of wet-back. This can reduce the amount of wet-back, especially for high-viscosity liquids that do not easily penetrate the water-absorbent resin layer. While it was thought that a top sheet with a low basis weight would be better for permeability to high-viscosity liquids, it turns out that a top sheet with a high basis weight can actually reduce the amount of wet-back.

[0019] [1. Definitions of Terms] [1-1. Water-absorbing resin] The term "water-absorbent resin" as used herein refers to a water-swellable and water-insoluble polymer gelling agent that satisfies the following physical properties: "Water-swellable" refers to a polymer gelling agent that has a CRC of 5 g / g or more as defined in ERT441.2-02, and "water-insoluble" refers to a polymer gelling agent that has an Ext of 50 wt% or less as defined in ERT470.2-02.

[0020] The water-absorbing resin can be appropriately designed depending on its application and is not particularly limited, but is preferably a hydrophilic cross-linked polymer obtained by cross-linking an unsaturated monomer having a carboxyl group. Moreover, the water-absorbing resin is not limited to a form in which the entire amount (100% by weight) is a polymer, and may be in the form of a water-absorbing resin composition containing additives and the like within a range that satisfies the physical properties (CRC, Ext).

[0021] Furthermore, the water-absorbent resin in the present invention is not limited to a final product, but may refer to an intermediate in the manufacturing process of the water-absorbent resin (for example, a hydrogel-like crosslinked polymer after polymerization, a dried polymer after drying, a water-absorbent resin powder before surface crosslinking, etc.), and all of these, together with the water-absorbent resin composition, are collectively referred to as "water-absorbent resin". Note that, examples of the shape of the water-absorbent resin include sheet-like, fibrous, film-like, particulate, gel-like, etc., but particulate water-absorbent resins are preferred in the present invention.

[0022] [1-2. "EDANA" and "ERT"] "EDANA" is an abbreviation for European Disposables and Nonwovens Associations, and "ERT" is an abbreviation for EDANA Recommended Test Methods, a European standard (almost a global standard) for measuring the properties of water-absorbent resins. In the present invention, unless otherwise specified, the physical properties of a water-absorbent resin are measured in accordance with the original ERT (revised in 2002 / publicly known document).

[0023] [1-3.Other] In this specification, the range "X to Y" means "X or more and Y or less."

[0024] In this specification, unless otherwise specified, "ppm" means "ppm by mass."

[0025] In this specification, the unit of volume "liter" may be written as "l" or "L".

[0026] In this specification, "weight" and "mass," "wt %" and "mass %," and "parts by weight" and "parts by mass" are treated as synonyms.

[0027] In this specification, "acid (salt)" means "acid and / or its salt." "(Meth)acrylic" means "acrylic and / or methacrylic." "Polyacrylic acid (salt)-based water-absorbing resin" means a water-absorbing resin containing a repeating unit derived from acrylic acid (salt) as a main component, and specifically refers to a water-absorbing resin containing preferably 50 to 100 mol %, more preferably 70 to 100 mol %, still more preferably 90 to 100 mol %, and particularly preferably substantially 100 mol % of acrylic acid (salt) in the total monomers (excluding crosslinking agents) used in polymerization.

[0028] [2. Physical properties of water-absorbent resin] "Shape of water-absorbent resin" The water-absorbent resin contained in the absorbent article according to one embodiment of the present invention (hereinafter also referred to as "the water-absorbent resin of the present invention") is preferably in the form of particles, and specific examples thereof include irregularly crushed particles, spherical particles, football-shaped particles, aggregated particles, etc. Among these, spherical particles have a high bulk density, and aggregated particles have an improved water absorption rate, so the water-absorbent resin is preferably in the form of aggregated particles of spherical particles (for example, spherical particles containing a polyacrylic acid (salt)-based water-absorbent resin). Here, "spherical" includes not only true spheres but also approximately spherical particles with an aspect ratio of 1.0 to 1.2.

[0029] "Additives contained" In the present invention, the water-absorbent resin may contain additives to exhibit various functions. Specific examples of the additives include surfactants, compounds having phosphorus atoms, oxidizing agents, organic reducing agents, inorganic reducing agents, water-insoluble inorganic fine particles, chelating agents, polyvalent metal salts, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, and thermoplastic fibers. The amount of the additives used (added amount) is appropriately determined depending on the application of the resulting water-absorbent resin, and is 5% by mass or less, preferably 3% by mass or less, and more preferably 1% by mass or less, based on the water-absorbent resin (e.g., water-absorbent resin powder). The lower limit is 0.001% by mass or more, preferably 0.01% by mass or more, based on the water-absorbent resin (e.g., water-absorbent resin powder). The compounds disclosed in "(5) Water-Insoluble Inorganic Fine Particles" of International Patent Publication No. 2011 / 040530 are applicable to the present invention as the water-insoluble inorganic fine particles. Among these water-insoluble inorganic fine particles, the inclusion of particularly hydrophilic fine particles, for example, silica (silicon dioxide) or hydrotalcite, is preferred because it improves the liquid compatibility of the water-absorbent resin (for example, water-absorbent resin particles), and enables the water-absorbent resin to absorb aqueous liquids in a short time when used in absorbent articles.

[0030] The amount of water-insoluble inorganic fine particles added is 0.01 to 5 parts by mass, more preferably 0.05 to 3 parts by mass, still more preferably 0.1 to 1 part by mass, and particularly preferably 0.2 to 0.5 parts by mass, relative to 100 parts by mass of the water-absorbent resin, from the viewpoint of improving the liquid compatibility of the water-absorbent resin (for example, water-absorbent resin particles).

[0031] "CRC" "CRC" is an abbreviation for Centrifuge Retention Capacity, which means the water absorption capacity of a water-absorbent resin under no pressure.

[0032] The CRC of the water-absorbent resin of the present invention is preferably 25 g / g or more, more preferably 28 g / g or more, and even more preferably 31 g / g or more. There is no particular limitation on the upper limit, and a higher CRC is preferable, but from the viewpoint of balance with other physical properties, it is preferably 50 g / g or less, more preferably 45 g / g or less, even more preferably 40 g / g or less, and particularly preferably 35 g / g or less.

[0033] If the CRC is 25 to 50 g / g, the absorption capacity is sufficient, and a decrease in the rate of absorption of body fluids such as urine and blood is prevented, making the material suitable for use in high-absorption-rate disposable diapers, etc. The CRC value can be controlled by changing the type and amount of the internal crosslinking agent, surface crosslinking agent, etc.

[0034] "Ext." "Ext" is an abbreviation for Extractables (water-soluble content) and refers to the amount of extractable content extracted from the water-absorbent resin. The water-soluble content is measured in accordance with the EDANA method (ERT470.2-02), and in some cases, the extraction time is changed from 16 hours to 1 hour, and the water-soluble content in this case is called "Ext (1hr)".

[0035] The Ext of the water-absorbent resin of the present invention is preferably 33% by mass or less, more preferably 30% by mass or less, and further preferably 27% by mass or less. The lower limit is 0% by mass or more, but from the viewpoint of the balance with other physical properties, it is preferably 2% by mass or more, more preferably 4% by mass or more.

[0036] When the Ext is 33% by mass or less, a decrease in the rate of absorption of body fluids such as urine and blood is prevented, and the material is suitable for use in high-absorption-rate disposable diapers, etc. The Ext value can be controlled by changing the type and amount of the polymerization initiator, internal crosslinking agent, surface crosslinking agent, etc., and can also be controlled by using a chain transfer agent in the polymerization step.

[0037] The Ext(1hr) of the water absorbent resin of the present invention is preferably 15% by mass or less, more preferably 10% by mass or less. The lower limit is 0% by mass or more, but from the viewpoint of the balance with other physical properties, it is preferably 1% by mass or more, more preferably 2% by mass or more.

[0038] When the Ext(1hr) exceeds 15 mass%, a polymer component extracted from the water absorbent resin during water absorption may cause an increase in the viscosity of the absorbed liquid. The value of Ext(1hr) is controlled not only by the control method of the Ext but also by the composition of the surface crosslinking agent aqueous solution and the mixing method with the water absorbent resin.

[0039] "AAP" "AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption capacity of a water-absorbent resin under pressure. AAP is measured in accordance with the EDANA method (ERT442.2-02). Specifically, a 0.9% by mass aqueous solution of sodium chloride is used, and 0.9 g of the water-absorbent resin is subjected to pressure of 2.06 kPa (21 g / cm) for 1 hour. 2 After swelling under a pressure of 0.3 psi, the AAP (absorbency against pressure) (unit: g / g) is measured.

[0040] From the viewpoint of water absorption properties when used in sanitary materials, the AAP of the water-absorbent resin of the present invention is preferably 20 g / g or more, more preferably 25 g / g or more. The upper limit of the AAP of the water-absorbent resin is not particularly limited, but is preferably 45 g / g or less.

[0041] "Moisture content" The "moisture content" is measured in accordance with the EDANA method (ERT430.2-02), except that the sample amount is changed to 1.0 g and the drying temperature is changed to 180°C.

[0042] The water content of the water-absorbent resin of the present invention is not particularly limited, but is preferably 1% by mass to 20% by mass, more preferably 1% by mass to 15% by mass, even more preferably 2% by mass to 13% by mass, and particularly preferably 2% by mass to 10% by mass. If the water content is 1% by mass to 20% by mass, a decrease in the rate at which the resin absorbs body fluids such as urine and blood is prevented, and the resin is suitable for use in high-absorption-rate disposable diapers and the like.

[0043] "Mass average particle diameter (D50)" The "mass-average particle diameter (D50)" is defined as "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation" in columns 27 and 28 of U.S. Patent No. 7,638,570. It is measured in accordance with the "(σζ) of Particle Diameter Distribution" standard.

[0044] The mass average particle diameter (D50) of the water-absorbent resin of the present invention is preferably 200 μm to 700 μm, more preferably 250 μm to 600 μm, even more preferably 250 μm to 500 μm, and particularly preferably 300 μm to 450 μm. The proportion of particles with a particle diameter of less than 150 μm is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. When the mass average particle diameter is 200 μm or more, dust is small and handling is easy. When the mass average particle diameter is 700 μm or less, a decrease in the rate of absorption of body fluids such as urine and blood is prevented, and the resin is suitable for use in high-absorption-rate disposable diapers and the like.

[0045] "Free swelling rate (A) in a 3 wt% polyethylene oxide aqueous solution at 40°C" "Free swelling rate (A) in a 3% by weight polyethylene oxide aqueous solution at 40°C" is the water absorption rate (unit: g g) when a water-absorbent resin absorbs 20 times its own weight in a 3% by weight polyethylene oxide aqueous solution at 40°C without pressure or stirring. -1 ·s -1 "Free swelling rate (A) in a 3 wt % aqueous polyethylene oxide solution at 40°C" is sometimes referred to as the free swelling rate (A).

[0046] The free swelling rate (A) of the water-absorbent resin of the present invention is 0.15 g g -1 ·s -1 More than 0.18g g -1 ·s -1 More than 0.20 g·g -1 ·s -1 More preferably, 0.25 g g -1 ·s -1 The upper limit of the free swelling rate (A) of the water-absorbent resin is not particularly limited, but is preferably 0.50 g g -1 ·s -1 The following is the result.

[0047] "Free swelling rate in saline at 40°C (B)" "Free swelling rate in saline solution at 40°C (B)" is the water absorption rate (unit: g g) when a water-absorbent resin absorbs 20 times its own weight in saline solution at 40°C (0.9 mass% sodium chloride aqueous solution) without pressure or stirring. -1 ·s -1 ) The "free swelling rate (B) in physiological saline at 40°C" is sometimes referred to as the free swelling rate (B).

[0048] The free swelling rate (B) of the water-absorbent resin of the present invention is 0.40 g g -1 ·s -1 More than 0.50 g g -1 ·s -1 More than 0.60 g g -1 ·s -1 The upper limit of the free swelling rate (B) of the water-absorbent resin is not particularly limited, but is preferably 2.00 g g-1 ·s -1 Less than or equal to 1.50 g·g -1 ·s -1 Less than 1.00 g g -1 ·s -1 The following is the result.

[0049] "Free swelling rate ratio" The "free swelling rate ratio" is the free swelling rate (A) [g g -1 ·s -1 ] / Free swelling rate (B) [g·g -1 ·s -1 ] can be calculated using

[0050] The free swelling rate ratio of the water absorbent resin of the present invention is 0.20 or more, preferably 0.25 or more, more preferably 0.30 or more. The upper limit of the free swelling rate ratio of the water absorbent resin is not particularly limited, but is preferably 1.00 or less, more preferably 0.60 or less, and even more preferably 0.40 or less.

[0051] "Bulk density" "Bulk density" is measured in accordance with the EDANA method (ERT460.2-02).

[0052] The bulk density of the water-absorbent resin of the present invention is preferably 0.40 g / cm 3 ~0.80g / cm 3 , more preferably 0.50 g / cm 3 ~0.80g / cm 3 , and more preferably 0.60 g / cm 3 ~0.75g / cm 3 The bulk density is 0.40 g / cm 3 ~0.80g / cm 3 If so, the decrease in the rate at which the absorbent absorbs body fluids such as urine and blood is prevented, and the absorbent is suitable for use in high-absorption-rate disposable diapers and the like.

[0053] "Number average particle size" When the water-absorbent resin is in the form of aggregates, the number average particle size of the primary particles constituting the aggregates is measured using an electron microscope. The number average particle size of the primary particles of the water-absorbent resin is preferably 5 to 1000 μm, more preferably 5 to 800 μm, even more preferably 8 to 500 μm, still more preferably 10 to 300 μm, still more preferably 10 to 200 μm, and particularly preferably 30 μm to 100 μm.

[0054] "surface tension" The surface tension of the water absorbent resin of the present invention is preferably 60 mN / m or more, more preferably 65 mN / m or more, further preferably 67 mN / m or more, particularly preferably 71 mN / m or more, and there is no substantial decrease in the surface tension. An upper limit of 75 mN / m is usually sufficient.

[0055] 3. Method for producing water-absorbent resin The method for producing the water absorbent resin of the present invention may be any of aqueous solution polymerization, reversed-phase suspension polymerization, gas-phase droplet polymerization, and other polymerization methods, but in view of ease of controlling the physical properties of the water absorbent resin of the present invention, reversed-phase suspension polymerization will be described below as an example. In particular, unlike general reversed-phase suspension polymerization which includes an azeotropic dehydration step in a hydrophobic organic solvent after polymerization and a surface crosslinking step in a dispersion system, a production method including a separation step of a reversed-phase suspension polymerization gel, a gel granulation step, a drying step (preferably hot air drying), and a surface crosslinking step (preferably powder surface treatment) will be described as an example.

[0056] In one embodiment of the present invention, the polymerization method may be any method that obtains a hydrogel polymer by reversed-phase suspension polymerization, in which droplets containing a monomer are dispersed or suspended in a liquid phase consisting of a hydrophobic organic solvent and then polymerized, and the polymerization method may be a batch method or a continuous method.

[0057] The batch production method is a production method in which an aqueous monomer solution is added or dropped into a hydrophobic organic solvent in a reaction apparatus and mixed to disperse or suspend the aqueous monomer solution, and then polymerization is carried out to obtain a hydrogel polymer.

[0058] On the other hand, the continuous production method is a method in which an aqueous monomer solution is continuously fed into a hydrophobic organic solvent in a reaction vessel, dispersed or suspended therein, and then polymerized, and the hydrogel polymer formed by the polymerization reaction and the hydrophobic organic solvent are continuously discharged from the reaction vessel.

[0059] A preferred embodiment of the present invention is continuous reversed-phase suspension polymerization, and more preferably liquid-phase continuous droplet polymerization in which an aqueous monomer solution is continuously dispersed in a hydrophobic organic solvent and polymerization is carried out. Such a continuous production process is preferable in that operations within and between steps can be continuously carried out, and mass production can be achieved through long-term operation. Furthermore, continuous reversed-phase suspension polymerization is a preferred form in terms of the physical properties of the water-absorbent resin.

[0060] The method for producing a water-absorbent resin according to the present invention may include a separation step for separating the hydrophobic organic solvent from the hydrogel polymer obtained in the polymerization step. In a continuous production process, it is preferable to recover the hydrophobic organic solvent separated from the hydrogel polymer in the separation step and reuse it as the hydrophobic organic solvent in the polymerization step. Such a circulatory production process can reduce the amount of organic solvent used, which is preferable in terms of production costs and waste liquid treatment.

[0061] Continuous polymerization is a process in which an aqueous monomer solution is continuously suspended or dispersed as droplets in a hydrophobic organic solvent in a dispersing device, and the resulting dispersion and / or suspension is continuously supplied to a reactor. This process is clearly distinguishable from a process in which dispersion and polymerization are carried out in a single device (batch operation, batch method). When continuous operation is carried out, the operating time is preferably 1 hour or more, more preferably 3 hours or more, even more preferably 8 hours or more, and even more preferably 24 hours or more. The operating time is usually 1 year or less.

[0062] The method for producing a water-absorbent resin according to the present invention includes an optional monomer aqueous solution preparation step, an optional dispersion step, a polymerization step, an optional separation step, an optional gel sizing step, and a drying step. After the drying step, the method may optionally include a cooling step, a pulverization step, a classification step, a surface cross-linking step, a rewetting step, a sizing step, a fine powder removal step, a granulation step, and a fine powder reuse step. The method may further include a transportation step, a storage step, a packaging step, a keeping step, and the like.

[0063] Each step will be described below.

[0064] [3-1: Monomer aqueous solution preparation process] The aqueous monomer solution is an aqueous solution containing a monomer that is a raw material for the water-absorbent resin, and is a solution that is dispersed or suspended in a hydrophobic organic solvent to carry out reversed-phase suspension polymerization.

[0065] The solvent for the aqueous monomer solution is preferably water or a mixture of water and a water-soluble organic solvent (e.g., alcohol, etc.), and more preferably water. In the case of a mixture of water and a water-soluble organic solvent, the water-soluble organic solvent (e.g., alcohol, etc.) is preferably 30% by mass or less, more preferably 5% by mass or less of the mixture.

[0066] As the monomer, a water-soluble ethylenically unsaturated monomer is preferably used. Examples of the water-soluble ethylenically unsaturated monomer include (meth)acrylic acid, (anhydrous) maleic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropane sulfonic acid, 2-(meth)acryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxyethyl (meth)acryloyl phosphate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, and other acid group-containing unsaturated monomers; (meth)acrylamide, N-ethyl (meth)acrylate, N-methyl ... Examples of the unsaturated monomers include amide group-containing unsaturated monomers such as acrylamide, N,N-dimethyl(meth)acrylamide, Nn-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine, and N-vinylacetamide; amino group-containing unsaturated monomers such as N,N-dimethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylamide, and N,N-diethylaminoethyl(meth)acrylate; mercapto group-containing unsaturated monomers; phenolic hydroxyl group-containing unsaturated monomers; and lactam group-containing unsaturated monomers such as N-vinylpyrrolidone.

[0067] In consideration of the stability of the water-soluble ethylenically unsaturated monomer, a polymerization inhibitor may be added to the aqueous monomer solution, if necessary.

[0068] Among the water-soluble ethylenically unsaturated monomers, when an acid group-containing unsaturated monomer having an acid group such as a carboxyl group is used to produce a water-absorbent resin, a neutralized salt in which the acid group is neutralized can be used. In this case, the salt of the acid group-containing unsaturated monomer is preferably a salt with a monovalent cation, more preferably at least one selected from alkali metal salts, ammonium salts, and amine salts, still more preferably an alkali metal salt, still more preferably at least one selected from sodium salts, lithium salts, and potassium salts, and particularly preferably a sodium salt.

[0069] Among these, from the viewpoint of the water absorption performance of the obtained water absorbent resin, the water-soluble ethylenically unsaturated monomer is preferably an acid group-containing unsaturated monomer and / or a salt thereof, more preferably (meth)acrylic acid (salt), maleic anhydride (salt), itaconic acid (salt), cinnamic acid (salt), still more preferably (meth)acrylic acid (salt), and particularly preferably acrylic acid (salt).

[0070] When an acid group-containing unsaturated monomer is used as a monomer, it is preferable to use it in combination with a neutralized salt of the acid group-containing unsaturated monomer from the viewpoint of the water absorption performance of the resulting water absorbent resin. From the viewpoint of water absorption performance, the number of moles of the neutralized salt relative to the total number of moles of the acid group-containing unsaturated monomer and its neutralized salt (hereinafter referred to as "neutralization rate") is preferably 40 mol% or more, more preferably 40 mol% to 95 mol%, even more preferably 50 mol% to 90 mol%, even more preferably 55 mol% to 85 mol%, and particularly preferably 60 mol% to 80 mol%.

[0071] In the production method according to the present invention, in preparing the aqueous monomer solution, any one of the above-exemplified monomers may be used alone, or any two or more monomers may be used in appropriate mixture. Furthermore, other monomers may also be used in mixture as long as the object of the present invention is achieved.

[0072] In the preparation of the aqueous monomer solution, when two or more kinds of monomers are used in combination, it is preferable to contain acrylic acid (salt) as a main component. In this case, the ratio of acrylic acid (salt) to the total monomers used for polymerization is usually 50 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more (upper limit 100 mol%), from the viewpoint of the water absorption performance of the obtained water absorbent resin.

[0073] In preparing the aqueous monomer solution, an internal crosslinking agent can be used as needed. Examples of the internal crosslinking agent include conventionally known internal crosslinking agents having two or more polymerizable unsaturated groups or two or more reactive groups in one molecule. Examples of the internal crosslinking agent include N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane di(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and the like. Examples of the internal crosslinking agent include (meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, glycidyl (meth)acrylate, etc. These internal crosslinking agents may be used alone or in combination of two or more.

[0074] Although it may be appropriately determined depending on the desired physical properties of the water absorbent resin, the amount of the internal crosslinking agent used is usually 0.0001 to 5 mol %, more preferably 0.001 to 3 mol %, and even more preferably 0.005 to 1.5 mol %, based on the monomer.

[0075] In addition, the following substances (hereinafter referred to as "other substances") may be added to the aqueous monomer solution.

[0076] Specific examples of other substances include chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines, and hypophosphites; blowing agents such as carbonates, bicarbonates, azo compounds, and bubbles; chelating agents such as metal salts of ethylenediaminetetraacetic acid and metal salts of diethylenetriaminepentaacetic acid; polyacrylic acid (salts) and crosslinked products thereof, starch, cellulose, starch-cellulose derivatives, polyvinyl alcohol, etc. The other substances may be used alone or in combination of two or more.

[0077] The amount of other substances used is not particularly limited, but the total concentration of the other substances is preferably 10% by mass or less, more preferably 1% by mass or less, and even more preferably 0.1% by mass or less, relative to the monomer. However, the total concentration of polyacrylic acid (salts) and their crosslinked products, starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 10% by mass or less, relative to the monomer.

[0078] The dissolved oxygen in the aqueous monomer solution may be reduced by raising the temperature or by replacing the oxygen with an inert gas.

[0079] "Polymerization initiator" A polymerization initiator may be used in preparing the aqueous monomer solution. When a polymerization initiator is used in preparing the aqueous monomer solution, gelation or an increase in viscosity of the aqueous monomer solution may occur. Therefore, it is preferable to add the polymerization initiator immediately before dispersing and / or suspending the aqueous monomer solution in the hydrophobic organic solvent, to cool the aqueous monomer solution and mix it with the polymerization initiator at a temperature lower than room temperature (20°C or lower, preferably around 0°C), or to subject the aqueous monomer solution and the polymerization initiator to the dispersion step while line mixing. A thermally decomposable polymerization initiator is preferably used as the polymerization initiator. The thermally decomposable polymerization initiator refers to a compound that decomposes when heated to generate radicals. From the viewpoints of the storage stability of the thermally decomposable polymerization initiator and the production efficiency of the water-absorbent resin, a water-soluble compound whose 10-hour half-life temperature is preferably 0°C to 120°C, more preferably 30°C to 100°C, and even more preferably 50°C to 80°C is preferably used as the polymerization initiator.

[0080] From the viewpoint of the handleability of the thermally decomposable polymerization initiator and the physical properties of the water absorbent resin, preferably an azo compound or a persulfate, more preferably sodium persulfate, potassium persulfate, or ammonium persulfate, further preferably sodium persulfate is used as the polymerization initiator.

[0081] The amount of the thermal decomposition type polymerization initiator used is appropriately set depending on the types of the monomer and the polymerization initiator, and is not particularly limited, but from the viewpoint of production efficiency, it is preferably 0.001 g / mol or more, more preferably 0.005 g / mol or more, and even more preferably 0.01 g / mol or more relative to the monomer. Moreover, from the viewpoint of improving the water absorption performance of the water absorbent resin, it is preferably 2 g / mol or less, more preferably 1 g / mol or less.

[0082] If necessary, other polymerization initiators such as a photodegradable polymerization initiator may be used in combination. Specific examples of the photodegradable polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, and benzophenone derivatives.

[0083] Furthermore, the thermally decomposable polymerization initiator can be used in combination with a reducing agent to form a redox polymerization initiator. In the redox polymerization initiator, the thermally decomposable polymerization initiator functions as an oxidizing agent. The reducing agent to be used is not particularly limited, but examples thereof include (bis)sulfites such as sodium sulfite and sodium hydrogen sulfite; reducing metal salts such as ferrous salts; L-ascorbic acid (salts), and amines.

[0084] "Monomer concentration in aqueous monomer solution" In the present invention, the concentration of the monomer in the aqueous monomer solution is selected depending on the selected monomer and the type of hydrophobic organic solvent, etc., but from the viewpoint of production efficiency, the lower limit is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more, of the aqueous monomer solution, and the upper limit is preferably 100% by mass or less, more preferably 90% by mass or less, even more preferably 80% by mass or less, and even more preferably 70% by mass or less.

[0085] As long as the object of the present invention is not impaired, it is also possible to blend additives such as an internal crosslinking agent, a surfactant, a density adjuster, a thickener, a chelating agent, etc. The type and amount of the additive can be appropriately selected depending on the combination of the monomer and the hydrophobic organic solvent used.

[0086] [3-2: Dispersion process] The dispersion process is a process of dispersing or suspending droplets containing a monomer in a hydrophobic organic solvent. Hereinafter, the term "dispersion" is intended to encompass suspension. More specifically, the aqueous monomer solution is added to a hydrophobic organic solvent and mixed and stirred to achieve dispersion. For example, a stirrer equipped with a stirring blade (such as a propeller blade, paddle blade, anchor blade, turbine blade, Pfaudle blade, ribbon blade, or flat blade) may be used. When using such a stirrer equipped with a stirring blade, the size of the dispersed droplets can be adjusted by the type, blade diameter, and rotation speed of the stirring blade, making it particularly suitable for use in batch-type reverse-phase suspension polymerization. Alternatively, a dispersion can be obtained by methods described in International Publication Nos. 2009 / 025235 and 2013 / 018571. When performing continuous reverse-phase suspension polymerization, the dispersion process preferably involves continuously supplying the aqueous monomer solution and the hydrophobic organic solvent separately to a disperser to produce droplets containing the monomer dispersed in the hydrophobic organic solvent.

[0087] When continuous reversed-phase suspension polymerization is carried out, examples of the dispersing device used in the dispersing step include, but are not particularly limited to, a spray nozzle, a high-speed rotary shear type agitator (rotary mixer type, turbo mixer type, disk type, double cylinder type, etc.), a cylindrical nozzle such as a needle, an orifice plate having a large number of holes directly formed in a plate, a spray nozzle, and a centrifugal atomizer such as a rotating wheel.

[0088] "Hydrophobic organic solvent" Preferred hydrophobic organic solvents include at least one organic solvent selected from the group consisting of aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. Specific examples include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane, and decalin; aromatic hydrocarbons such as benzene, toluene, and xylene; and halogenated hydrocarbons such as chlorobenzene, bromobenzene, carbon tetrachloride, and 1,2-dichloroethane. Among these, n-hexane, n-heptane, and cyclohexane are preferred from the viewpoints of availability and quality stability. A mixed solvent of two or more of these solvents can also be used.

[0089] In the present invention, a dispersing aid such as a surfactant or a polymer additive may be added to the hydrophobic organic solvent as needed, as long as the object of the present invention is not impaired. The type of dispersing aid is appropriately selected depending on the combination of the hydrophobic organic solvent and the monomer used, and examples of dispersing aids that can be used include the following surfactants and polymer additives.

[0090] Specific examples of the surfactant include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylaryl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, and phosphate esters of polyoxyethylene alkyl allyl ethers. Two or more of these surfactants may be used in combination. Polymerizable surfactants having polymerizability may also be used. Specific examples of the polymerizable surfactant include compounds having the following structure:

[0091] [ka]

[0092] In the formula, R 1 and R 2are each independently hydrogen, methyl, or ethyl, and n is an integer of 3 to 20. Among the surfactants described above, fatty acid esters such as sucrose fatty acid esters, polyglycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, and polyethylene glycol fatty acid esters are preferred, with sucrose fatty acid esters being particularly preferred. Furthermore, the HLB (hydrophilic-hydrophobic balance) of the surfactant used in the present invention is not particularly limited, but is preferably in the range of 1 to 20, more preferably 1 to 10, and even more preferably 3 to 6.

[0093] Specific examples of the polymer additive include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified ethylene-propylene-diene terpolymer (EPDM), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethyl cellulose, hydroxyethyl cellulose, etc. Among these, from the viewpoint of dispersion stability of the aqueous monomer solution, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer are preferred. Two or more of these may be used in combination. These polymer additives may also be used in combination with the surfactants. Among these, it is preferable to use a polymer additive, and it is more preferable to use a maleic anhydride-modified ethylene-propylene copolymer. In another preferred embodiment, the polymer additive is used alone without using a surfactant.

[0094] The amount of the dispersing aid used is appropriately set depending on the polymerization form, the types of the aqueous monomer solution and the hydrophobic organic solvent, etc. Specifically, the concentration of the dispersing aid in the hydrophobic organic solvent is preferably 0.0001 to 2% by mass, and more preferably 0.0005 to 1% by mass.

[0095] [3-3. Polymerization process] The polymerization step is a step in which the droplets containing the monomer obtained in the dispersion step are polymerized to obtain a hydrogel polymer (hereinafter also simply referred to as a hydrogel).

[0096] "Reactor" The reaction apparatus used in the polymerization step may be the same as the dispersing apparatus used in the dispersing step, or may be a different apparatus. In the case of batch-type reversed-phase suspension polymerization, the apparatus used in the dispersing step can be used as the reaction apparatus, which is preferable in terms of workability. When the reaction apparatus is a different apparatus from the dispersing apparatus, the monomer dispersion obtained in the dispersing step is supplied to the reaction apparatus.

[0097] The shape of the reactor in which the polymerization reaction is carried out is not particularly limited, and known reactors can be used. As described above, a stirring device that can be suitably used in the dispersion step can also be suitably used in the polymerization reaction. In the case of a continuous production method, the reactor is preferably shaped so that the monomer (aqueous solution) can undergo the polymerization reaction while moving as a droplet-like dispersed phase in the hydrophobic organic solvent that is the continuous phase formed in the reactor. Examples of such reactors include reactors in which tubular reaction tubes are arranged vertically, horizontally, or spirally. In this embodiment, the monomer (aqueous solution) is supplied into the hydrophobic organic solvent moving within the reaction section, so that droplets of the monomer aqueous solution move together with the hydrophobic organic solvent without stagnating. This prevents contact between monomer reactants with different polymerization rates.

[0098] Furthermore, the reaction apparatus may be equipped with a temperature control means so that the continuous phase inside the reaction apparatus can be heated or cooled from the outside, as necessary.

[0099] "Polymerization temperature" The polymerization temperature, which is the reaction temperature in the polymerization step, may be set appropriately depending on the type and amount of polymerization initiator used, but is preferably 20°C to 100°C, and more preferably 40°C to 90°C. Polymerization temperatures higher than 100°C are not preferred because a rapid polymerization reaction occurs. The polymerization temperature refers to the temperature of the hydrophobic organic solvent, which is the dispersion medium (hereinafter referred to as "Td").

[0100] In the polymerization step, since the monomer (aqueous solution) is dispersed in the hydrophobic organic solvent in the form of droplets, the temperature of the aqueous monomer solution is rapidly increased by heat transfer from the hydrophobic organic solvent. If the polymerization initiator contained in the droplets is a thermally decomposable polymerization initiator, the thermally decomposable polymerization initiator decomposes as the temperature rises, generating radicals. The generated radicals then initiate a polymerization reaction, and as the polymerization reaction progresses, a hydrogel is formed.

[0101] In the case of a continuous production method, the formed hydrogel moves inside the reactor with the moving continuous phase and is discharged from the reactor together with the hydrophobic organic solvent that forms the continuous phase.

[0102] When the aqueous monomer solution contains a thermally decomposable polymerization initiator, the Td is, from the viewpoint of the polymerization rate, preferably 70° C. or higher, more preferably 75° C. or higher, and even more preferably 80° C. or higher. There are no particular limitations on the upper limit of Td, but from the viewpoint of safety, it is appropriately selected within a range that does not exceed the boiling point of the hydrophobic organic solvent that forms the continuous phase.

[0103] "Multi-stage reversed-phase suspension polymerization" In the production method of the present invention, from the viewpoint of obtaining an appropriate aggregate particle size, multi-stage polymerization may be performed. Specifically, multi-stage polymerization can be performed by, for example, adding an aqueous monomer solution after the completion of the first polymerization stage and carrying out a polymerization reaction.

[0104] "Inorganic fine particles" In the production method of the present invention, inorganic fine particles may be added to the hydrogel polymer during and / or after the polymerization in order to obtain an appropriate aggregate particle size.

[0105] Examples of inorganic fine particles that can be used in the present invention include silicon dioxide, amorphous silica, aluminum oxide, titanium dioxide, calcium phosphate, calcium carbonate, magnesium phosphate, calcium sulfate, diatomaceous earth, bentonite, zeolite, and other metal oxides. Silicon dioxide, aluminum oxide, and titanium dioxide are particularly preferred.

[0106] The amount of inorganic fine particles added is generally 0.001 to 1 part by weight, preferably 0.001 to 0.5 parts by weight, relative to the hydrogel polymer, to obtain good results. This range is preferred because the effects of adding inorganic fine particles are efficiently expressed and there is little effect on water absorption performance.

[0107] [3-4. Separation process] The separation step is a step of separating the hydrogel polymer obtained in the polymerization step from the hydrophobic organic solvent. The type and structure of the apparatus used in the separation step are not particularly limited, and for example, known apparatuses used for filtration, sedimentation, centrifugation, squeezing, etc. can be used. Alternatively, the hydrogel polymer and the hydrophobic organic solvent may be separated by heating the mixture of the hydrogel polymer and the hydrophobic organic solvent at normal or reduced pressure using the stirring device having stirring blades used in the polymerization step and distilling the mixture. In batch-type reversed-phase suspension polymerization, distillation at normal or reduced pressure is preferably carried out.

[0108] "Solvent content of hydrophobic organic solvent in hydrogel" The content of the hydrophobic organic solvent in the hydrogel separated from the hydrophobic organic solvent through the separation step is not particularly limited, but from the viewpoint of the load during drying and the cost of the organic solvent, the content of the hydrophobic organic solvent (hereinafter also simply referred to as solvent content) is preferably 0.01 to 10 mass %, more preferably 0.01 to 9 mass %, and even more preferably 0.01 to 5 mass %, relative to 100 mass % of the hydrogel including the solvent.

[0109] "Solid content of hydrogel polymer" The solid content of the hydrogel separated from the hydrophobic organic solvent is not particularly limited. However, from the viewpoint of the drying cost in the subsequent drying step, it is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. Furthermore, from the viewpoint of water absorption performance and mechanical load, the upper limit of the solid content of the hydrogel polymer is preferably 90% by mass or less, more preferably 80% by mass or less, even more preferably 70% by mass or less, and particularly preferably 60% by mass or less.

[0110] "Gel polymerization rate" The gel polymerization rate of the obtained hydrogel is preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, from the viewpoint of suppressing aggregation of the obtained hydrogel during drying and reducing residual monomers in the obtained water-absorbent resin. The upper limit of the gel polymerization rate is ideally 100% by mass. By setting the polymerization rate to 70% by mass or more, it is possible to suppress strong aggregation of the hydrogels and formation of clumps during drying.

[0111] [3-5. Gel granulation process] In the gel sizing step, the hydrogel polymer separated from the hydrophobic organic solvent in the separation step is sized using a gel sizing device having an extrusion section and a perforated plate. This results in a sized hydrogel polymer (hereinafter, the hydrogel after gel sizing will be referred to as sized gel). The gel sizing step is an optional step. By including the gel sizing step, it becomes easier to control the water absorption rate of a high-viscosity liquid.

[0112] The hydrogel polymer subjected to this gel sizing step is in the form of a single gel sphere or an aggregate of gel spheres. The lower limit of the average particle size of the hydrogel polymer is not particularly limited, but is preferably 0.01 mm or more, more preferably 0.03 mm or more, even more preferably 0.05 mm or more, and even more preferably 0.1 mm or more. The upper limit is also not particularly limited, but is preferably 20 mm or less, more preferably 10 mm or less. Furthermore, the primary particle size refers to the particle size of a single particle, and to the particle size of each gel sphere constituting the aggregate, if the polymer is in an aggregate form. In the present invention, the average primary particle size is not particularly limited, but from the viewpoint of suppressing the generation of fine powder when controlling the particle size of the final product, it is preferably 5 to 2000 μm, more preferably 5 to 1000 μm, even more preferably 5 to 800 μm, even more preferably 8 to 500 μm, even more preferably 10 to 300 μm, and particularly preferably 10 to 200 μm.

[0113] In addition, a device having a cutter may be installed before the gel particle size adjusting device having the extrusion section and the perforated plate to break down large aggregates.

[0114] "Gel granulation device" As used herein, "gel sizing" refers to the process of producing particles of approximately uniform shape and size from a wet powder raw material by extruding the wet powder mass through small holes in a perforated plate into a cylindrical shape. In other words, by using a perforated plate, the hydrogel that has become excessively coarse aggregates in the preceding solvent separation step is broken down, and the hydrogel in the form of small single particles is appropriately aggregated. Therefore, this step makes it possible to obtain a hydrogel (sized gel) in a granulated form with a relatively uniform particle size. The sized gel may contain single-particle hydrogel.

[0115] The "gel sizing device having an extrusion section and a perforated plate" used in the gel sizing step is not particularly limited as long as it has an extrusion section and a perforated plate (die or screen), the extrusion section usually has an extrusion member that extrudes and supplies the contents toward the perforated plate, and is an apparatus (e.g., an extruder) that can produce particles of a certain size by extruding the material through the perforated plate. These apparatuses may also be used in series.

[0116] Furthermore, the shape of the holes in the perforated plate (die or screen) is not particularly limited and can be arbitrarily selected from shapes suitable for use, such as perfect circles, ellipses, polygons (e.g., hexagons), and triangles. However, perfect circles and ellipses are preferred from the viewpoint of sizing strength. The hole diameter is also not particularly limited, but is preferably 1.5 mm or less, more preferably 1.0 mm or less, and even more preferably 0.8 mm or less. By keeping the hole diameter below this upper limit, the size of the resulting sieved gel is prevented from increasing more than necessary, and the amount of fine powder generated during particle size control of the final product can be reduced. The hole diameter is preferably 0.3 to 1.5 mm, more preferably 0.3 to 0.8 mm. A hole diameter of 0.3 mm or more in the perforated plate ensures efficient extrusion during the extrusion operation. The hole diameter is defined as follows: First, if the holes are not perfect circles, the geometric mean value of the minor and major axes of the holes is used as the hole diameter. In addition, when the hole diameters of the perforated plate are different, the hole diameters of all the holes are calculated, and the arithmetic mean value is adopted as the hole diameter of the perforated plate. Furthermore, when the hole diameter of the perforated plate changes from the extrusion action side to the opposite side (the hole diameter changes in the thickness direction of the perforated plate), the value with the smallest hole diameter among them is adopted.

[0117] In this step, further additives may be added. Examples of additives that can be added in this step include polymerization initiators, oxidizing agents, reducing agents, chelating agents, thickeners, surfactants, crosslinking agents, acids, bases, foaming agents, organic or inorganic fine particles, and polyvalent metal salts. Among these, additives that can control the degree of aggregation, such as thickeners such as starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol, surfactants, fine powder of water-absorbent resins, crosslinking agents, and polyvalent metal salts, are preferred.

[0118] [3-6. Drying process] The drying step is a step of drying the hydrogel. This removes the water contained in the hydrogel and the hydrophobic organic solvent that was not completely separated in the optional separation step, thereby obtaining a particulate dry polymer having a desired solid content. The solid content of the dry polymer is preferably 80% by weight or more, more preferably 85 to 99% by weight, even more preferably 90 to 98% by weight, and particularly preferably 92 to 97% by weight.

[0119] In the present invention, the drying method is not particularly limited, and examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, drying by azeotropic dehydration with a hydrophobic organic solvent, and high-humidity drying using high-temperature water vapor.

[0120] The drying temperature (hot air temperature) in the hot air drying is preferably 100 to 250° C., more preferably 100 to 180° C., from the viewpoint of the color tone of the water-absorbent resin and drying efficiency. Drying conditions other than the drying temperature, such as the hot air speed and drying time, may be appropriately set depending on the water content and total weight of the particulate hydrogel to be dried and the target resin solid content, and when band drying is performed, the conditions described in WO 2006 / 100300, WO 2011 / 025012, WO 2011 / 025013, WO 2011 / 111657, etc. are appropriately applied.

[0121] The dried polymer made of particles obtained in this drying step can be used as a water-absorbent resin for various applications as it is. When a water-absorbent resin is produced by this manufacturing method, the dried polymer obtained in the drying step can also be subjected to a surface-crosslinking step described later. In this case, the dried polymer subjected to the surface-crosslinking step described later is also referred to as a "water-absorbent resin powder" for convenience. When a dried product in which particles of the granulated gel are aggregated in the drying step is obtained, it is preferable that the drying step include a step of loosening the aggregates.

[0122] "Additives" As long as the effects of the present invention are not impaired, additives may be added to the hydrogel. The addition may be carried out during heating by a heating means or before the drying step (before heating by a heating means). Furthermore, the additive may be added at any step before the drying step. The additive can reduce excessive adhesion between hydrogel particles during drying, thereby making it possible to obtain a water-absorbent resin with an excellent water absorption rate.

[0123] [3-7. Hydrophilic treatment process] In the present invention, the hydrophilization treatment step may be carried out after or before the surface cross-linking step described below. The hydrophilization treatment step is a step of hydrophilizing the water-absorbent resin with an organic solvent. This can enhance the effect of the surface cross-linking treatment (increasing the absorption capacity of the water-absorbent resin under pressure and reducing the amount of resorption). In addition, the increased hydrophilicity of the surface of the water-absorbent resin also increases the water absorption rate.

[0124] The specific hydrophilization treatment method varies depending on the cause of the formation of the hydrophobic portion on the surface of the water absorbent resin. For example, when the water absorbent resin is produced by reverse phase suspension polymerization, the surfactant used as a dispersant during polymerization is the cause of the hydrophobicity, so it is preferable to wash it off, and more preferably to wash it off with an organic solvent.

[0125] The organic solvent is not particularly limited as long as it can wash the surfactant, but in order to enhance the washing effect, it is preferable to use one that does not cause the water-absorbent resin to swell. The preferred swelling ratio of the water-absorbent resin during treatment is less than 2 times. As the organic solvent, not only hydrophilic organic solvents but also hydrophobic organic solvents can be used. Specific examples of hydrophilic organic solvents include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol; ketones such as acetone; ethers such as dioxane and tetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxides such as dimethyl sulfoxide. Examples of hydrophobic organic solvents include aliphatic hydrocarbons such as n-pentane, n-heptane, n-hexane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane, and decalin; halogenated hydrocarbons such as chlorobenzene, bromobenzene, carbon tetrachloride, and 1,2-dichloroethane; and aromatic hydrocarbons such as benzene, toluene, and xylene. Among these, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-heptane, n-hexane, and cyclohexane are preferably used.

[0126] In the present invention, when a water-absorbent resin is subjected to a hydrophilization treatment using an organic solvent, it is preferable to bring the organic solvent into contact with the water-absorbent resin in a heated state, as this may further improve the absorption capacity under pressure and the resorption amount. The heating temperature is preferably below the boiling point of the organic solvent, and is generally about 40 to 120°C, depending on the type of organic solvent used. The hydrophilization treatment is preferably performed while the water-absorbent resin is dry. Care must be taken when performing the hydrophilization treatment while the resin is wet with an organic solvent or water, as this may result in a decrease in the water absorption rate or a sudden decrease in the water absorption capacity when crosslinking is performed near the surface. Therefore, it is preferable to filter and dry the water-absorbent resin obtained by reversed-phase suspension polymerization or aqueous solution polymerization to remove the polymerization solvent and water, and then perform the hydrophilization treatment.

[0127] [3-8.Surface crosslinking process] The water-absorbent resin (for example, water-absorbent resin powder) obtained through the drying step (and any subsequent step) is preferably surface-crosslinked with a surface crosslinking agent. This surface crosslinking is a treatment for providing a portion with high crosslink density in the surface layer of the water-absorbent resin (for example, water-absorbent resin powder) (a portion several tens of μm from the surface of the water-absorbent resin (for example, water-absorbent resin powder)). By carrying out the surface crosslinking treatment, various water-absorbing properties can be improved. Note that, in the present invention, known surface crosslinking techniques are applied as appropriate. Note that the surface crosslinking agent used in this step is also referred to as a "post-crosslinking agent" in known techniques in order to distinguish it from the internal crosslinking agent used in the step of preparing an aqueous monomer solution.

[0128] In the present invention, the surface cross-linking step may be carried out after the drying step or during the drying step. In known surface cross-linking steps, a surface cross-linking agent is generally mixed with a hydrogel cross-linked polymer or a cross-linked polymer of the dried product thereof, and the mixture is heated to carry out a cross-linking reaction. However, in the present invention, these steps may be separately provided after the drying step, or a surface cross-linking agent may be added in the drying step to carry out the surface cross-linking reaction and drying simultaneously. Furthermore, when a water-absorbent resin is produced by a batch-type reversed-phase suspension polymerization method, the solvent and the hydrogel polymer can be separated by distillation in the separation step after the polymerization reaction, but a surface-cross-linked water-absorbent resin (e.g., water-absorbent resin particles) can be obtained by adding a surface cross-linking agent even during the separation step.

[0129] [3-9. Other processes] The method for producing a water-absorbent resin according to the present invention may include, in addition to the above-mentioned steps, a cooling step, a pulverizing step, a water-containing (rewetting) step, a classification step, a step of adding other additives, a sizing step, and a fine powder recycling step, as needed. In addition, the method may further include a transporting step, a storing step, a packaging step, a preservation step, etc.

[0130] (cooling process) In the cooling step, which is optionally performed, the particulate dry polymer obtained in the drying step can be cooled using a known cooling means to obtain a particulate dry polymer cooled to a desired temperature.

[0131] (Crushing process) It is preferable to carry out a pulverization step in which the particulate dried polymer obtained in the drying step (and any subsequent cooling step) is pulverized. By carrying out the pulverization step, a water absorbent resin powder having a controlled particle size or particle size distribution is obtained.

[0132] In the pulverization step, a suitable pulverization means may be selected from high-speed rotary pulverizers such as roll mills, hammer mills, screw mills, and pin mills, vibration mills, knuckle-type pulverizers, and cylindrical mixers.

[0133] (Rewetting process) This step, which is optionally carried out, is a step of adding at least one kind of additive selected from the group consisting of a cationic polymer, a chelating agent, an inorganic reducing agent, and an α-hydroxycarboxylic acid compound to the water absorbent resin (e.g., water absorbent resin particles) obtained in the surface cross-linking step.

[0134] The additive is preferably added to a water-absorbent resin (for example, water-absorbent resin particles) in the form of an aqueous solution or a dispersion (slurry). The additive may be added and mixed simultaneously with the above-mentioned surface cross-linking agent solution. Specifically, the method described in "(2-7) Rewetting step" in International Patent Publication No. 2015 / 053372 is also applied to the present invention.

[0135] (Other additive addition process) In the present invention, additives other than those mentioned above can be added to the water-absorbent resin to impart various functions. Specific examples of such additives include surfactants, compounds containing phosphorus atoms, oxidizing agents, organic reducing agents, inorganic reducing agents, water-insoluble inorganic fine particles, chelating agents, polyvalent metal salts, organic powders such as metal soaps, deodorants, antibacterial agents, pulp, and thermoplastic fibers. The water-insoluble inorganic fine particles disclosed in International Patent Publication No. 2011 / 040530, “[5] Water-insoluble inorganic fine particles,” are applicable to the present invention. Among these additives, the addition of water-insoluble inorganic fine particles, particularly hydrophilic fine particles such as silica (silicon dioxide), is preferred because it improves the liquid compatibility of the water-absorbent resin (e.g., water-absorbent resin particles) and allows the resin to absorb aqueous liquids in a short time when used in absorbent articles.

[0136] (Sizing process) The "size regulating step" means a step of loosening a water absorbent resin that has been loosely aggregated through the surface cross-linking step, and regulating the particle size. This size regulating step includes a fine powder removing step and a classification step that follow the surface cross-linking step. The size regulating step is preferably carried out from the viewpoint of regulating the particle size of the water absorbent resin and obtaining stable water absorption properties.

[0137] (Fine powder reuse process) The "fine powder reuse step" means a step of supplying fine powder generated by sieve classification or the like in each of the steps to any of the steps as it is or after granulating the fine powder. The fine powder reuse step is preferably carried out from the viewpoint of reducing production loss of a water absorbent resin.

[0138] 4. Absorbent articles and manufacturing methods thereof An absorbent article according to one embodiment of the present invention comprises, in this order, a liquid-permeable top sheet, an absorbent body containing the water-absorbent resin of the present invention, and a liquid-impermeable back sheet, and the water-absorbent resin has a basis weight of 100 to 1000 g / m 2 and the top sheet has a basis weight of 25 g / m 2 That's all.

[0139] A method for producing an absorbent article according to one embodiment of the present invention includes a step of disposing an absorbent body containing the water-absorbent resin of the present invention between a liquid-permeable top sheet and a liquid-impermeable back sheet, and the water-absorbent resin has a basis weight of 100 to 1000 g / m. 2 and the top sheet has a basis weight of 25 g / m 2 That's all.

[0140] A method for producing an absorbent article according to one embodiment of the present invention includes the steps of selecting the water-absorbent resin of the present invention and disposing an absorbent body containing the selected water-absorbent resin between a liquid-permeable top sheet and a liquid-impermeable back sheet, wherein the basis weight of the water-absorbent resin is 100 to 1000 g / m. 2 and the top sheet has a basis weight of 25 g / m 2 It may be more than that.

[0141] The absorbent article according to one embodiment of the present invention is not particularly limited, but preferred examples include disposable diapers (for infants and adults), sanitary napkins, incontinence pads, etc. In particular, the absorbent article according to one embodiment of the present invention can be used as a high-density disposable diaper.

[0142] In one embodiment of the present invention, the basis weight of the water-absorbent resin means the weight of the water-absorbent resin per unit area of ​​the absorbent article. The basis weight of the water-absorbent resin also means the weight of the water-absorbent resin per unit area of ​​the absorbent body constituting the absorbent article. When the basis weight of the water-absorbent resin is equal to or greater than a specific value, the absorption performance of the absorbent article is suitably improved, and the rewet amount when absorbing low-viscosity body fluids such as urine, and high-viscosity body fluids such as blood and loose stools, can be suitably reduced. From the viewpoint of suitably reducing the rewet amount, the basis weight of the water-absorbent resin is set to 100 g / m 2 More preferably, 150 g / m 2 More preferably, 180 g / m 2In addition, by making the basis weight of the water-absorbent resin equal to or less than a specific value, it is possible to prevent the absorbent article from becoming excessively thick and large. From the viewpoint of preventing the absorbent article from becoming excessively thick and large, the basis weight of the water-absorbent resin is set to 1000 g / m 2 Preferably, it is 500 g / m or less. 2 More preferably, it is 300 g / m or less. 2 The following is the result.

[0143] In one embodiment of the present invention, the top sheet can be any known sheet that can be used as a liquid-permeable sheet constituting an absorbent article, and is not particularly limited. The liquid-permeable top sheet is typically a nonwoven fabric, such as an air-through nonwoven fabric, a point-bonded nonwoven fabric, a spunbonded nonwoven fabric, or a spunlace nonwoven fabric. Furthermore, composite nonwoven fabrics of these can also be used as the liquid-permeable top sheet. Examples of composite nonwoven fabrics include spunbond / meltblown / meltblown / spunbonded nonwoven fabrics (SMMS nonwoven fabrics). The liquid-permeable top sheet is preferably a spunlace nonwoven fabric. These nonwoven fabrics are preferably hydrophilized with a surfactant.

[0144] The material constituting the top sheet is not particularly limited, and examples thereof include synthetic fibers such as polypropylene, polyethylene, and polyester.

[0145] In the present invention, the basis weight of the top sheet is the weight per unit area of ​​the top sheet.

[0146] The basis weight of the top sheet is 25 g / m 2 More preferably, it is 30 g / m 2 More preferably, 35 g / m 2 The basis weight of the top sheet is preferably 70 g / m from the viewpoint that the top sheet itself can retain the liquid excessively absorbed in the absorbent article and prevent the liquid from returning to the skin side when a load is applied. 2More preferably, it is 60 g / m or less. 2 The following is the result.

[0147] In one embodiment of the present invention, the back sheet may be any known sheet that can be used as a liquid-impermeable sheet constituting an absorbent article, and is not particularly limited. The liquid-impermeable back sheet may be, for example, a thin plastic film such as a polyethylene film. Among such plastic films, a breathable film is preferred in order to provide comfort to the user of the absorbent article.

[0148] An absorbent body according to one embodiment of the present invention includes the water-absorbent resin. The absorbent body may also include a hydrophilic fiber material and / or an additive in addition to the water-absorbent resin. The additive may be any additive that is generally contained in absorbents in absorbent articles, and is not particularly limited. Specific examples of the additive include inorganic powders (e.g., amorphous silica), deodorants, pigments, dyes, antibacterial agents, fragrances, and adhesives. When the water-absorbent resin contains inorganic particles, the absorbent body may also contain an inorganic powder in addition to the inorganic particles in the water-absorbent resin. Examples of the inorganic powder include silicon dioxide, zeolite, kaolin, and clay.

[0149] The method for preparing the absorbent body in one embodiment of the present invention can be any method commonly known for preparing absorbent bodies for absorbent articles, and is not particularly limited. Specific examples of the method for preparing the absorbent body include a method of mixing the water-absorbent resin of the present invention with a hydrophilic fibrous material to obtain a mixture and molding the mixture; a method of forming a layer of hydrophilic fibers and then spraying the water-absorbent resin of the present invention on the layer of hydrophilic fibers; and a method of spraying the water-absorbent resin of the present invention in layers to form a layer of absorbent resin and then placing a layer of hydrophilic fibers on the layer of water-absorbent resin. The absorbent body prepared in this manner may be a uniform mixture of the water-absorbent resin of the present invention and the hydrophilic fibers, or the water-absorbent resin of the present invention and the hydrophilic fibers may each form a layer and be in contact with each other. When the water-absorbent resin of the present invention and the hydrophilic fibers each form a layer, the water-absorbent resin of the present invention and the hydrophilic fibers may each form one layer, or at least one of the water-absorbent resin of the present invention and the hydrophilic fibers may form multiple layers, with the layers of the water-absorbent resin of the present invention and the layers of the hydrophilic fibers being alternately stacked. Even when the water-absorbent resin of the present invention and the hydrophilic fibers form separate layers, the water-absorbent resin of the present invention may be mixed into the hydrophilic fiber layer. In the absorbent core of the absorbent article according to one embodiment of the present invention, it is preferable to have a hydrophilic fiber layer on the back sheet side and a water-absorbent resin layer on the top sheet side, because the water-absorbent resin will be mixed into the hydrophilic fiber layer when the absorbent article is in use, from the viewpoints of the thickness of the absorbent article and the diffusibility of absorbed aqueous liquid.

[0150] The hydrophilic fibrous material is not particularly limited, and examples thereof include pulp fiber, cotton linter crosslinked cellulose fiber, rayon, cotton, wool, acetate, vinylon, etc. Furthermore, in one embodiment of the present invention, the hydrophilic fibrous material is preferably an air-laid hydrophilic fibrous material listed above.

[0151] The hydrophilic fiber material may be an absorbent material such as pulp fiber. In this case, the content of the water-absorbent resin in the absorbent body (core concentration) is preferably 30% by mass or more and less than 100% by mass, more preferably 40% by mass or more and less than 100% by mass, even more preferably 50% by mass or more and less than 100% by mass, still more preferably 60% by mass or more and less than 100% by mass, particularly preferably 70% by mass or more and less than 100% by mass, and most preferably 75% by mass or more and less than 95% by mass. However, in the present invention, the hydrophilic fibers used in the absorbent body do not include cloth-like materials (woven fabrics or nonwoven fabrics).

[0152] By setting the core concentration within the above range, when the absorbent body is used in the upper layer portion of an absorbent article, the absorbent article can be maintained in a clean white state. Furthermore, since the absorbent body has excellent diffusibility of body fluids such as urine and blood, efficient liquid distribution is achieved, and an improvement in the absorption capacity of the absorbent article according to one embodiment of the present invention can be expected.

[0153] More specifically, the absorbent body in one embodiment of the present invention may be an absorbent body that contains a hydrophilic fibrous material in addition to the water-absorbent resin in one embodiment of the present invention, and the mass of the water-absorbent resin is 50% by mass or more and less than 100% by mass of the total mass of the water-absorbent resin and the hydrophilic fibrous material.

[0154] The mass of the water-absorbent resin may be 50% by mass or more of the total mass of the water-absorbent resin and the hydrophilic fibrous material, more specifically, the mass of the water-absorbent resin may be 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, less than 100% by mass, or 100% by mass of the total mass of the water-absorbent resin and the hydrophilic fibrous material.

[0155] In the method for producing an absorbent article according to one embodiment of the present invention, the method for selecting the water-absorbent resin of the present invention is not particularly limited, and for example, a method can be mentioned in which a part of the water-absorbent resin is divided, and the free swelling rate (A) and the free swelling rate (B) of the divided part of the water-absorbent resin are measured, and after it is confirmed that the water-absorbent resin corresponds to the water-absorbent resin of the present invention, the remaining water-absorbent resin is used as a raw material for the absorbent article. [Example]

[0156] The present invention will be described in more detail with reference to the following examples and comparative examples. However, the present invention is not limited to these examples, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention.

[0157] Unless otherwise noted, the electrical equipment used in the Production Examples, Examples, and Comparative Examples (including measurements of the physical properties of the water-absorbent resin) used a power supply of 200 V or 100 V at 60 Hz. Furthermore, unless otherwise noted, the physical properties of the water-absorbent resin produced in this Production Example and the absorbent articles obtained in this Example and this Comparative Example were measured under conditions of room temperature (20 to 25°C) and a relative humidity of 50% RH.

[0158] For convenience, "liter" may be written as "l" or "L," and "weight %" as "wt%." Furthermore, when measuring trace components, values ​​below the detection limit are written as "ND" (Non Detected).

[0159] [Measurement of physical properties of water-absorbent resin] "CRC" CRC (centrifuge retention capacity) was measured in accordance with the EDANA method (ERT441.2-02). Specifically, 0.2 g of the water-absorbent resin was placed in a nonwoven bag, and then immersed in a large excess of 0.9% by mass sodium chloride aqueous solution for 30 minutes to allow free swelling. Thereafter, the bag was centrifuged (250 G) for 3 minutes to drain the water, and the water absorption capacity (unit: g / g) was determined.

[0160] "Ext" The Ext (water-soluble content) of the water-absorbent resins of the present Examples and Comparative Examples was measured in accordance with the EDANA method (ERT470.2-02).

[0161] "Ext(1hr)" The Ext (1 hr) of the water-absorbent resins of the present Examples and Comparative Examples was measured in accordance with the EDANA method (ERT470.2-02). Note that the stirring time was changed to 1 hour.

[0162] "AAP" The AAP (absorbency under pressure) was measured in accordance with the EDANA method (ERT442.2-02).

[0163] "Moisture content" The moisture content was measured in accordance with the EDANA method (ERT430.2-02). For the measurement, the mass of the sample (water-absorbent resin) was changed to 1.0 g, the drying temperature to 180°C, and the drying time to 3 hours. Specifically, 1.0 g of the water-absorbent resin was placed in an aluminum cup with a bottom diameter of 50 mm, and then the total mass W1 (g) of the sample (water-absorbent resin and aluminum cup) was accurately weighed. Next, the sample (water-absorbent resin and aluminum cup) was placed in an oven with an ambient temperature set to 180°C. After 3 hours, the sample was removed from the oven, and the total mass W2 (g) was accurately weighed. When the mass of the sample (water-absorbent resin) used in this measurement was M (1.0 g), the moisture content α (mass%) of the sample was calculated according to the following (Equation 1): Moisture content α (mass%) = {(W1-W2) / M}×100 Equation (1).

[0164] "Mass average particle diameter (D50)" The mass-average particle diameter (D50) was measured according to the method described in columns 27 and 28 of U.S. Pat. No. 7,638,570, under "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution."

[0165] "Free swelling rate (A) in a 3 wt% polyethylene oxide aqueous solution at 40°C" Polyethylene oxide (PEO-1, viscosity-average molecular weight 150,000-400,000, manufactured by Sumitomo Seika Chemicals Co., Ltd.) was used to prepare an aqueous polyethylene oxide solution with a concentration of 3 wt % and a viscosity of 10±1 mPa·s at a liquid temperature of 40°C. A vibration viscometer (model: VM-10A) manufactured by SEKONIC Corporation was used to measure the viscosity. 25 ml and 50 ml glass beakers were kept at 40°C for use below.

[0166] 0.50 g of the water-absorbent resin was placed in a 25 ml glass beaker (inner diameter 32 to 34 mm, height 50 mm) and the weight was measured (W3). At this time, the upper surface of the water-absorbent resin placed in the beaker was made horizontal. If necessary, the upper surface of the water-absorbent resin was made horizontal by carefully tapping the beaker or other measures.

[0167] A glass funnel capable of pouring a 3 wt% polyethylene oxide aqueous solution at a flow rate of 5-6 g / sec was placed in the center of the beaker containing the water-absorbent resin, with the tip of the funnel positioned 50 mm above the bottom of the beaker. Next, 10 g of a 3 wt% polyethylene oxide aqueous solution that had been adjusted to 40°C ± 0.5°C was weighed into a 50 ml glass beaker and carefully and quickly poured into the funnel.

[0168] Time measurement was started at the same time that the 3 wt% polyethylene oxide aqueous solution poured into the funnel came into contact with the water-absorbent resin. After the 3 wt% polyethylene oxide aqueous solution was poured into the funnel, the funnel was removed when no water droplets fell from the funnel for 5 seconds. Immediately after the start of time measurement, the upper surface of the water-absorbent resin was present below the liquid level of the 3 wt% polyethylene oxide aqueous solution in the beaker.

[0169] Then, when the liquid level of the 3 wt % polyethylene oxide aqueous solution in the beaker into which the 3 wt % polyethylene oxide aqueous solution had been poured was visually observed at an angle of approximately 20°, time measurement was ended (unit: second) (tS1) at the time point when the liquid level was replaced by the upper surface of the water absorbent resin that had absorbed the liquid (the time point when the shape of gel particles on the upper surface of the water absorbent resin could be recognized and no liquid remained in the center of the upper surface of the water absorbent resin).

[0170] Next, the weight (unit: g) of the 25 ml glass beaker after the 3 wt % aqueous polyethylene oxide solution was poured into it was measured (W4).

[0171] The weight (W5, unit: g) of a 3 wt % polyethylene oxide aqueous solution poured into a 25 ml glass beaker was determined using the following formula (a). The free swelling rate (A) was calculated using the following formula (b).

[0172] Formula (a): W5(g) = W4(g) - W3(g), Equation (b): Free swelling rate (A) [g g -1 ·s -1 ]=W5 / (tS1 × mass of absorbent resin (g)).

[0173] "Free swelling rate in 0.9% by mass sodium chloride aqueous solution (B)" The viscosity of a 0.9% by mass aqueous solution of sodium chloride at 40°C was measured using a vibration viscometer (model: VM-10A) manufactured by SEKONIC Corporation, and was found to be 0 to 1 mPa·s. Below, 25 ml and 50 ml glass beakers were used, kept at 40°C.

[0174] 1.00 g of the water-absorbent resin was placed in a 25 ml glass beaker (inner diameter 32 to 34 mm, height 50 mm). At this time, the top surface of the water-absorbent resin placed in the beaker was made horizontal. If necessary, the top surface of the water-absorbent resin was made horizontal by carefully tapping the beaker or other measures.

[0175] Next, 20 g of a 0.9 mass % sodium chloride aqueous solution adjusted to 40°C ± 0.5°C was weighed into a 50 ml glass beaker, and the total weight (unit: g) of the 0.9 mass % sodium chloride aqueous solution and the glass beaker was measured (W6). The entire weighed amount of the 0.9 mass % sodium chloride aqueous solution was carefully and quickly poured into the 25 ml beaker containing the water-absorbent resin.

[0176] Time measurement was started at the same time as the 0.9% by mass aqueous sodium chloride solution poured into the beaker came into contact with the water-absorbent resin. Immediately after the start of time measurement, the upper surface of the water-absorbent resin was present below the liquid level of the 0.9% by mass aqueous sodium chloride solution in the beaker. When the liquid level of the 0.9% by mass aqueous sodium chloride solution in the beaker into which the 0.9% by mass aqueous sodium chloride solution had been poured was visually observed at an angle of about 20°, the time measurement was ended (unit: seconds) (tS2) at the time when the liquid level was replaced by the upper surface of the absorbent resin that had absorbed the liquid (the time when the shape of the gel particles on the upper surface of the water-absorbent resin could be recognized and no liquid remained in the center of the upper surface of the water-absorbent resin).

[0177] Next, the weight (unit: g) of the 50 ml glass beaker after the 0.9 mass % sodium chloride aqueous solution was poured into it was measured (W7).

[0178] The weight (W8, unit: g) of the 0.9 mass % sodium chloride aqueous solution poured into a 25 ml glass beaker was calculated using the following formula (c).

[0179] The free swelling rate (B) was calculated by the following formula (d).

[0180] Formula (c): W8(g) = W6(g) - W7(g), Equation (d): Free swelling rate (B) [g g -1 ·s -1 ]=W8 / (tS2 × mass of absorbent resin (g)).

[0181] "Free swelling rate ratio" The ratio of free swelling rates was calculated according to the following formula: Free swelling rate ratio = Free swelling rate (A) [g g -1·s -1 ] / Free swelling rate (B) [g·g -1 ·s -1 ].

[0182] "Bulk density" The bulk density of the water-absorbent resin of the present invention was measured in accordance with the EDANA method (ERT460.2-02).

[0183] "Number average particle size" A scanning electron microscope (SEM) photograph of the water-absorbent resin or water-absorbent resin powder was taken. 50 primary particles on the front surface of the aggregate-like particles were randomly selected from the photograph, and the major axis and minor axis of each primary particle were measured, and the average value of the measured values ​​was taken as the primary particle diameter. The average value of the primary particle diameters was calculated, and this average value was taken as the average primary particle diameter of the water-absorbent resin.

[0184] "viscosity" The viscosity of the present invention was measured using a vibration viscometer (model: VM-10A) manufactured by SEKONIC Co., Ltd. Specifically, 40 g to 45 g of the test liquid was placed in a screw tube (No. 7, 50 ml, code 730-09) manufactured by Maruemu Co., Ltd., the liquid temperature was adjusted to 40°C, and the height was adjusted so that the liquid surface was immersed 2 mm to 3 mm into the rod part at the top of the detector, and the viscosity (mPa s) was measured.

[0185] "surface tension" The surface tension in the present invention is the surface tension of an aqueous solution when a water-absorbent resin is dispersed in a 0.9 mass % aqueous sodium chloride solution, and was measured by the method described in WO2015 / 129917.

[0186] "Top sheet basis weight" Using the area and weight of the top sheet used in the examples and comparative examples described below, the basis weight of the top sheet was calculated according to the following formula (2). Top sheet weight (g) / Top sheet area (m 2 ) = Top sheet basis weight (g / m 2 ) (2) "Basis weight of water-absorbent resin" Using the area and weight of the absorbent body used in the examples and comparative examples described below, and the content (core concentration) of the water-absorbent resin in the absorbent body, the basis weight of the water-absorbent resin in the absorbent article was calculated based on the following formula (3). {Weight of absorbent material (g) × content of water-absorbent resin (% by weight) × 10 -2} / Area of ​​absorber (m 2 ) = basis weight of water-absorbent resin (g / m 2 ) (3) "Rewet" Using the test liquids (A) and (B) shown below, the "rewet amount (Rewet)" of the absorbent articles obtained in the Examples and Comparative Examples was measured by the method shown in <Method for measuring rewet amount> below. <Test solution> Test solution (A): A polyethylene oxide aqueous solution prepared using polyethylene oxide (PEO-1, viscosity average molecular weight 150,000-400,000, manufactured by Sumitomo Seika Chemicals Co., Ltd.), with a concentration of 3% by weight, a liquid temperature of 40°C, and a viscosity of 10±1 mPa·s under these conditions. Test liquid (B): Physiological saline solution (0.9% by mass sodium chloride aqueous solution) at a liquid temperature of 40°C <Method for measuring the amount of return> 1. A funnel (with an inner diameter of 6 mm at the tip) was placed so that its tip was 1 cm above the surface of the central part of the absorbent article. 20 g of test liquid at 40°C was poured into the central part of the absorbent article through the funnel, and the time when the test liquid was poured in was considered to be the time when the absorbent body and the liquid came into contact, and a timer was started. 2. 10 seconds after the test liquid was poured in, a kitchen paper (Oji Nepia Co., Ltd., product name: Super Absorbent Kitchen Towel) was placed on the absorbent article, and a plate (8 x 16 cm, 34.0 g) and a weight (455 g) were placed on the kitchen paper to measure the absorbency of 3.8 g / cm 3 The paper towel was left standing for 1 minute while applying a pressure of 1000 kJ / min, allowing the test liquid seeping out of the absorbent article to be absorbed into the paper towel. The weight of the paper towel was measured before placing it on the absorbent article. The measured weight of the paper towel was designated as the weight A (g) of the paper towel before absorbing water. After leaving the absorbent article for 3.1 minutes to allow the kitchen paper to absorb the test liquid that had seeped out of the absorbent article, the plate and the weight were removed, and the kitchen paper was then taken out and weighed. The weight of the kitchen paper taken out was designated as the weight of the kitchen paper after water absorption: B (g).

[0187] Using the weight of the kitchen paper before absorbing water: A (g) and the weight of the kitchen paper after absorbing water: B (g) obtained in steps 2 and 3, the amount of rewet was calculated based on the following formula (4). Rewet (g) = Weight of paper towel after absorbing water: B (g) - Weight of paper towel before absorbing water: A (g) (4) [Preparation of water-absorbent resin] [Manufacturing Example 1] A hydrogel polymer was prepared according to the manufacturing process shown in Figure 1 of WO 2020 / 067310.

[0188] The dispersion device used was a two-fluid spray nozzle (external mixing type, spray nozzle inner diameter: 0.5 mm, model: SETO07507S303+TS303, manufactured by Ikeuchi Co., Ltd.) shown in Figure 9 of International Publication No. 2020 / 067310, and the reaction device used was a vertically arranged PFA tube (inner diameter: 25 mm, total length: 10 m).

[0189] In preparation for the polymerization reaction, a solution of 0.005 mass% sucrose fatty acid ester (trade name: DK Ester (registered trademark) F-50 / manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., HLB=6) added to n-heptane (density: 0.68 g / ml) as a hydrophobic organic solvent was introduced into the auxiliary fluid flow path (second supply pipe) of the two-fluid spray nozzle, the reactor, the separator, and the piping connecting these. The position of the two-fluid spray nozzle was adjusted so that the tip of the two-fluid spray nozzle was immersed in the continuous phase consisting of the hydrophobic organic solvent contained in the reactor.

[0190] Next, the liquid feed pump was operated to start circulating the organic solvent at a flow rate of 1000 ml / min. In this production method, the route of the circulated organic solvent was branched into a route for feeding the organic solvent into the reactor via a two-fluid spray nozzle and a route for feeding the organic solvent directly into the reactor. The flow rate of the organic solvent fed into the reactor via the two-fluid spray nozzle was set to 800 ml / min, and the flow rate of the organic solvent fed directly into the reactor was set to 200 ml / min. The flow rate of the organic solvent at the tip of the two-fluid spray nozzle was 7.86 m / sec. The heat exchanger was also operated to heat the circulating organic solvent to a set temperature of 85°C.

[0191] Next, acrylic acid, a 48.5% by mass aqueous solution of sodium hydroxide, and ion-exchanged water were mixed, and then N,N-methylenebisacrylamide and trisodium diethylenetriaminepentaacetate were added to prepare a monomer solution (1). Separately, sodium persulfate and ion-exchanged water were mixed to prepare a 10% by mass aqueous solution of sodium persulfate (1).

[0192] Next, the monomer solution (1) and the sodium persulfate aqueous solution (1) obtained in the above procedure were separately fed into a mixer and mixed to prepare a monomer aqueous solution (1). The monomer concentration of the monomer aqueous solution (1) was 43 mass %, and the neutralization rate was 70 mol %. The internal crosslinking agent N,N-methylenebisacrylamide was 0.015 mol % relative to the monomer, the chelating agent diethylenetriaminepentaacetic acid trisodium was 100 ppm relative to the monomer, and the polymerization initiator sodium persulfate was 0.1 g / mol relative to the monomer.

[0193] Next, the aqueous monomer solution (1) prepared in the mixer was quickly sent to the aqueous monomer solution flow path (first supply pipe) of the two-fluid spray nozzle. Thereafter, the aqueous monomer solution (1) was supplied to the reactor together with the organic solvent at a flow rate of 40 ml / min (23.6 g / min) using the two-fluid spray nozzle. The aqueous monomer solution (1) was supplied in the same direction (cocurrent) as the circulation direction of the organic solvent forming the continuous phase. The flow rate of the aqueous monomer solution (1) at the tip of the two-fluid spray nozzle was 0.85 m / sec. The temperature of the aqueous monomer solution (1) before being supplied to the two-fluid spray nozzle was maintained at 25°C.

[0194] The aqueous monomer solution (1) supplied by the two-fluid spray nozzle was dispersed in the continuous phase in the form of droplets. The ratio of the aqueous monomer solution (1) to the organic solvent constituting the continuous phase (W / O ratio) was 3.3% by volume.

[0195] The dispersion thus obtained was then fed into a reactor, and the droplets of the aqueous monomer solution (1) polymerized while falling through the reactor filled with the hydrophobic organic solvent, which was the continuous phase, and minute spherical hydrogel polymers (1) were observed near the outlet of the reactor.

[0196] The hydrogel polymer (1) obtained by the above series of operations was continuously supplied together with the hydrophobic organic solvent from the reactor through a junction to a separator, where the hydrogel polymer (1) and the organic solvent were separated. In the separator, the hydrogel polymer (1) was an aggregate of minute spherical particles, and the size of the aggregates was 5 to 10 mm.

[0197] A hydrous gel polymer (1) (gel temperature: 90°C) was introduced into an extruder-type gel sizing device having a screw and a perforated plate with a hole diameter of 0.8 mm, and the hydrous gel polymer (1) was discharged from the gel sizing device to obtain a sizing gel (1).

[0198] Subsequently, the particulate hydrogel polymer (1) was dried by passing hot air at 105° C. for 45 minutes, to obtain particulate dried polymer (1).

[0199] Subsequently, the dried polymer (1) was supplied to a roll mill (WML type roll crusher, manufactured by Inokuchi Giken Co., Ltd.) and crushed to adjust the particle size, and further classified using a sieve having an opening particle size of 150 μm, to obtain a water absorbent resin powder (1).

[0200] 20 g of the water-absorbent resin powder (1) was added to 1000 ml of methanol heated to 60° C., stirred for 1 hour, filtered, and dried to carry out a hydrophilization treatment.

[0201] A surface crosslinking agent solution composed of 0.015 part by mass of ethylene glycol diglycidyl ether, 1.0 part by mass of propylene glycol and 3.0 parts by mass of ion-exchanged water was sprayed with a sprayer onto 100 parts by mass of the water absorbent resin powder (1), and the mixture was uniformly mixed using a high-speed continuous mixer.

[0202] The obtained mixture was introduced into a heat treatment machine whose atmospheric temperature was adjusted to 195°C ± 2°C, and subjected to heat treatment for 40 minutes, and then the powder temperature was forcibly cooled to 60°C, thereby obtaining a surface-crosslinked water absorbent resin powder (1). Hereinafter, the surface-crosslinked water absorbent resin powder is referred to as "water absorbent resin particle".

[0203] To 100 parts by mass of the water absorbent resin particles (1), a mixed liquid consisting of 0.40 parts by mass of a 27.5 wt% aqueous aluminum sulfate solution (8 wt% in terms of aluminum oxide), 0.134 parts by mass of a 60 wt% aqueous sodium lactate solution, and 0.002 parts by mass of propylene glycol was added. After the addition, the mixture was dried at 60°C for 30 minutes under windless conditions, and water was further added so that the moisture content of the water absorbent resin particles (1) became 10 wt%, and the mixture was passed through a JIS standard sieve with a mesh size of 1000 μm to obtain a water absorbent resin (1). The physical properties of the obtained water absorbent resin (1) are shown in Table 1.

[0204] [Manufacturing Example 2] A water absorbent resin (2) was obtained by the same procedure as in Production Example 1, except that (i) the addition of 0.005 mass% of a dispersing aid sucrose fatty acid ester (trade name: DK Ester (registered trademark) F-50 / manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., HLB=6) was changed to 0.005 mass% of a maleic anhydride-modified ethylene-propylene copolymer (trade name: Hiwax (registered trademark) 1105A / manufactured by Mitsui Chemicals, Inc.), and (ii) the internal crosslinking agent N,N-methylenebisacrylamide was changed to 0.015 mol% relative to the monomers, and polyethylene glycol diacrylate (average degree of polymerization: 9) was changed to 0.008 mol% relative to the monomers. The physical properties of the obtained water absorbent resin (2) are shown in Table 1.

[0205] [Manufacturing Example 3] 500 ml of n-heptane was added to a 1000 ml five-necked cylindrical round-bottom flask equipped with a stirrer, reflux condenser, dropping funnel, thermometer, and nitrogen gas inlet tube. 0.92 g of sucrose fatty acid ester (surfactant: S-370 manufactured by Mitsubishi Chemical Corporation) with an HLB of 3.0 was added and dispersed. The temperature was raised to dissolve the surfactant, and the mixture was then cooled to 55°C.

[0206] Separately, 92 g of an 80 wt % aqueous acrylic acid solution was added to a 500 mL Erlenmeyer flask. While cooling externally, 102.2 g of a 30 wt % aqueous sodium hydroxide solution was added dropwise to neutralize 75 mol % of the acrylic acid, preparing an aqueous solution of a partially neutralized acrylic acid. Furthermore, 50.2 g of water, 0.11 g of the polymerization initiator potassium persulfate, and 9.2 mg of the crosslinking agent ethylene glycol diglycidyl ether were added to prepare an aqueous monomer solution for the first-stage polymerization.

[0207] The entire amount of this aqueous monomer solution for first-stage polymerization was added to the five-neck cylindrical round-bottom flask while stirring at a stirrer speed of 500 rpm, and dispersed. The atmosphere inside the system was thoroughly purged with nitrogen, and then the temperature was increased. The bath temperature was maintained at 70°C, and the polymerization reaction was carried out for 1 hour, after which the polymerized slurry liquid was cooled to room temperature.

[0208] Furthermore, 119.1 g of an 80 wt % aqueous acrylic acid solution was placed in another 500 ml Erlenmeyer flask, and 132.2 g of a 30 wt % aqueous sodium hydroxide solution was added dropwise while cooling to neutralize 75 mol % of the acrylic acid. Further, 27.4 g of water, 0.14 g of potassium persulfate, and 35.7 mg of ethylene glycol diglycidyl ether were added to prepare an aqueous monomer solution for second-stage polymerization, which was then cooled in an ice-water bath.

[0209] While stirring at a stirrer speed of 1000 rpm, the entire amount of this aqueous monomer solution for the second-stage polymerization was added to the polymerization slurry. The system was then thoroughly purged with nitrogen again, and the temperature was raised. The bath temperature was maintained at 70°C, and the second-stage polymerization reaction was carried out for 2 hours. After polymerization was completed, 0.53 g of a 40 wt% aqueous solution of pentasodium diethylenetriaminepentaacetate as an aminocarboxylic acid-based metal chelating agent was added to the hydrous gel dispersed in n-heptane with stirring. Then, 266 g of water was extracted from the hydrous gel by azeotropic dehydration. 8.44 g of a 2 wt% aqueous solution of ethylene glycol diglycidyl ether was added to the resulting gel, and the water and n-heptane were removed by distillation, followed by drying, to obtain a dried polymer. This dried polymer was passed through a sieve with an opening of 850 μm, and 0.1 mass % of amorphous silica (manufactured by Oriental Silicas Corporation, Tokusil (registered trademark) NP-S) relative to the mass of the dried polymer was mixed with the dried polymer to obtain a comparative water absorbent resin (1) containing amorphous silica. The physical properties of the obtained comparative water absorbent resin (1) are shown in Table 1.

[0210] [Manufacturing Example 4] A monomer aqueous solution was prepared by mixing 67.0 parts of a 37% aqueous solution of sodium acrylate, 10.2 parts of acrylic acid, 0.079 parts of polyethylene glycol diacrylate (average number of ethylene oxide units: 8), and 22.0 parts of water. Nitrogen was blown into the monomer aqueous solution in a tray to adjust the dissolved oxygen concentration in the solution to 0.1 ppm or less.

[0211] The temperature of the aqueous monomer solution was then adjusted to 18°C ​​under a nitrogen atmosphere. Then, 0.16 parts of a 5% aqueous solution of sodium persulfate, 0.16 parts of a 5% aqueous solution of 2,2'-azobis(2-amidinopropane) dihydrochloride, 0.15 parts of a 0.5% aqueous solution of L-ascorbic acid, and 0.17 parts of a 0.35% aqueous solution of hydrogen peroxide were added dropwise in this order with stirring. Polymerization began immediately after the addition of hydrogen peroxide. Stirring was then stopped, and the monomer temperature reached its peak temperature 10 minutes later. The peak temperature was 85°C. The tray was then immersed in an 80°C water bath and aged for 10 minutes. The resulting transparent hydrogel was crushed with a meat chopper and then dried at 180°C for 30 minutes.

[0212] The dried product was pulverized in a pulverizer and classified into particles that passed through a 500 μm sieve and remained on a 105 μm sieve, thereby obtaining a water absorbent resin powder. 100 parts of the water absorbent resin powder was mixed with a composition liquid consisting of 0.002 parts of diethylenetriaminepentaacetic acid, 0.05 parts of ethylene glycol diglycidyl ether, 1 part of propylene glycol, 3 parts of water, and 1 part of isopropyl alcohol, and the mixture was heat-treated at 180° C. for 40 minutes, thereby obtaining comparative water absorbent resin particles (2).

[0213] Subsequently, 0.3 mass% of silicon dioxide (Aerosil (registered trademark) 200, manufactured by Nippon Aerosil Co., Ltd.) was added as an inorganic powder to 100 parts of the comparative water absorbent resin particles (2) to obtain a comparative water absorbent resin (2). The physical properties of this comparative water absorbent resin (2) were measured, and the results are shown in Table 1.

[0214] [Table 1]

[0215] From Table 1, it was found that the water-absorbent resins (1) and (2) prepared in Production Examples 1 and 2 correspond to the water-absorbent resins of the present invention. On the other hand, it was found that the comparative water-absorbent resins (1) and (2) prepared in Production Examples 3 and 4 do not correspond to the water-absorbent resins of the present invention.

[0216] [Manufacture of absorbent articles] [Example 1] A desalination type vinyl tape manufactured by Nitto Denko was cut into a sheet of 10 cm wide and 18 cm long, and the resulting sheet was used as a liquid-impermeable back sheet. 2 A liquid-permeable top sheet was prepared by cutting a sheet (materials: polypropylene, polyethylene) into a size of 10 cm wide and 18 cm long. 0.5 g of ground pulp, a hydrophilic fiber material, was uniformly placed on the adhesive side of a rectangular section of the liquid-impermeable back sheet, measuring 8 cm wide and 16 cm long, 1 cm inward from each of the four edges. Next, 2.56 g of water-absorbent resin (1) (basis weight 200 g / m) was placed on top of the ground pulp. 2 ) was evenly spread, and a 1 kg weight with a base measuring 8 cm wide and 16 cm long was placed on the area where the ground pulp and the water-absorbent resin were placed, and pressure was applied for 1 minute. After that, the weight was removed, and the liquid-permeable top sheet was sandwiched between the two sheets and attached to the four edges up to 1.0 cm inward, to produce an absorbent article. The produced absorbent article was designated absorbent article (1).

[0217] [Example 2] As a liquid-permeable top sheet, a spunbond nonwoven fabric (basis weight: 40 g / m) was used instead of a sheet obtained by cutting the spunlace nonwoven fabric into a size of 10 cm wide and 18 cm long. 2 An absorbent article was manufactured in the same manner as in Example 1, except that a sheet obtained by cutting a sheet of polyester (materials: polypropylene, polyethylene) into a size of 10 cm wide and 18 cm long was used. The manufactured absorbent article was designated absorbent article (2).

[0218] [Example 3] An absorbent article was produced in the same manner as in Example 1, except that the water-absorbent resin (2) prepared in Production Example 2 was used instead of the water-absorbent resin (1). The produced absorbent article was designated as absorbent article (3). [Comparative Example 1] An absorbent article was produced in the same manner as in Example 1, except that the comparative water-absorbent resin (1) prepared in Production Example 3 was used instead of the water-absorbent resin (1). The produced absorbent article was designated as comparative absorbent article (1). Comparative Example 2 An absorbent article was produced in the same manner as in Example 1, except that the comparative water-absorbent resin (2) prepared in Production Example 4 was used instead of the water-absorbent resin (1). The produced absorbent article was designated as comparative absorbent article (2). Comparative Example 3 By changing the amount of the water-absorbent resin (1) used to 1.02 g, the basis weight of the water-absorbent resin in the absorbent body, i.e., the basis weight of the water-absorbent resin in the obtained absorbent article, was changed to 80 g / m 2 Except for the above change, an absorbent article was produced in the same manner as in Example 1. The produced absorbent article was designated as comparative absorbent article (3). Comparative Example 4 By changing the amount of the water-absorbent resin (2) used to 0.21 g, the basis weight of the water-absorbent resin in the absorbent body, i.e., the basis weight of the water-absorbent resin in the obtained absorbent article, was changed to 16 g / m 2 Except for the above change, an absorbent article was produced in the same manner as in Example 3. The produced absorbent article was designated as comparative absorbent article (4). Comparative Example 5 By changing the amount of the comparative water-absorbent resin (1) to 0.21 g, the basis weight of the water-absorbent resin in the absorbent body, i.e., the basis weight of the water-absorbent resin in the obtained absorbent article, was increased to 16 g / m 2 Except for the above change, an absorbent article was produced in the same manner as in Comparative Example 1. The produced absorbent article was designated comparative absorbent article (5). Comparative Example 6 As a liquid-permeable top sheet, the spunbond nonwoven fabric (basis weight: 40 g / m 2 Instead of a sheet obtained by cutting a sheet of polypropylene and polyethylene (10cm wide and 18cm long), a spunbond nonwoven fabric (13g / m 2 An absorbent article was manufactured in the same manner as in Example 2, except that a sheet obtained by cutting a sheet of polyester (materials: polypropylene, polyethylene) into a size of 10 cm wide and 18 cm long was used. The manufactured absorbent article was designated comparative absorbent article (6).

[0219] [result] The rewet of the absorbent articles (1) to (3) produced in Examples 1 to 3 and the comparative absorbent articles (1) to (6) produced in Comparative Examples 1 to 6 was measured by the method described above. The results are shown in Table 2 below.

[0220] [Table 2]

[0221] As shown in Table 2, the absorbent articles produced in Examples 1 to 3, which contain a water-absorbent resin corresponding to the water-absorbent resin of the present invention, have less rewet in both the test liquid (A) corresponding to a high-viscosity body fluid and the test liquid (B) corresponding to a low-viscosity body fluid, than the absorbent articles produced in Comparative Examples 1 and 2, which contain a water-absorbent resin not corresponding to the water-absorbent resin of the present invention. Therefore, it was found that the absorbent article according to one embodiment of the present invention, by containing the water-absorbent resin of the present invention, has a smaller rewet amount when absorbing a low-viscosity body fluid and a high-viscosity body fluid.

[0222] The basis weight of the water-absorbent resin is 100 to 1000 g / m 2 The absorbent articles manufactured in Examples 1 and 3 have a basis weight of the water-absorbent resin of 100 g / m 2 The rewet in both test liquid (A) and test liquid (B) is less than that of the absorbent articles manufactured in Comparative Examples 3 and 4, which are outside the above-mentioned range. Therefore, the absorbent article according to one embodiment of the present invention has a basis weight of the water-absorbent resin of 100 to 1000 g / m 2 It was found that by keeping the viscosity within this range, the amount of return when absorbing low-viscosity and high-viscosity body fluids was reduced.

[0223] The above-mentioned items include water-absorbent resins that do not fall under the category of the water-absorbent resin of the present invention, and the water-absorbent resin has a basis weight of 100 to 1000 g / m 2 This can also be understood from the fact that the absorbent article manufactured in Comparative Example 5, which is outside the range, had more Rewet in both test liquid (A) and test liquid (B) than the absorbent articles manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 and 6.

[0224] Top sheet weight: 25g / m 2 The absorbent article manufactured in Example 2, which has the above values, has a top sheet basis weight of 25 g / m 2 The rewet in the test liquid (A) is less than that of the absorbent article produced in Comparative Example 6. Therefore, the absorbent article according to one embodiment of the present invention has a top sheet basis weight of 25 g / m 2 From the above, it was found that the absorbency rate of highly viscous body fluids was also excellent, and the amount of resorption when absorbing highly viscous body fluids was small.

[0225] In view of the above, an absorbent article according to one embodiment of the present invention comprises the water-absorbent resin of the present invention, and the water-absorbent resin has a basis weight of 100 to 1000 g / m 2 and the weight of the top sheet is 25 g / m 2 As a result of the above, it has been found that the absorbent has an excellent absorption rate not only for low-viscosity body fluids (e.g., urine) but also for high-viscosity body fluids (e.g., a mixture of urine and loose stools), and has the effect of reducing the amount of return when absorbing the aforementioned body fluids.

[0226] The absorbent article produced in Example 1, which used a spunlace nonwoven fabric as the top sheet, had less rewet than the absorbent article produced in Example 2, which used a spunbond nonwoven fabric as the top sheet having the same basis weight as the top sheet described in Example 1. This demonstrates that a spunlace nonwoven fabric is preferable for the top sheet. [Industrial Applicability]

[0227] The present invention can be used in applications such as disposable diapers (for infants and adults), sanitary napkins, and incontinence pads.

Claims

1. An absorbent article comprising, in this order, a liquid-permeable top sheet, an absorbent body containing a water-absorbent polymer, and a liquid-impermeable back sheet, The water-absorbent resin has a basis weight of 100 to 1000 g / m ２ and The top sheet has a basis weight of 25 g / m ２ That's all, The water-absorbing resin is a polyacrylic acid (salt)-based water-absorbing resin that satisfies the following requirements (a) to (c): An absorbent article, wherein the polyacrylic acid (salt)-based water-absorbing resin contains 50 mol % or more of structural units derived from acrylic acid and sodium acrylate relative to the total structural units derived from monomers: (a) A free swelling rate (A) of 0.15 g g in an aqueous polyethylene oxide solution at 40°C －１ ・s －１ or more and 0.50 g·g −1 ·s −1 or less; (b) Free swelling rate (B) in physiological saline at 40°C is 0.40 g g －１ ・s －１ or more and 2.00 g·g −1 ·s −1 or less; (c) The ratio of free swelling rate (A) / free swelling rate (B) is 0.20 or more and 1.00 or less.

2. The absorbent article according to claim 1, wherein the water-absorbent resin has a CRC of 25 to 50 g / g.

3. The absorbent article according to claim 1 or 2, wherein the water-absorbent resin is an aggregate of spherical particles.

4. The bulk density of the water-absorbent resin is 0.40 to 0.80 g / cm ３ The absorbent article according to any one of claims 1 to 3,

5. The free swelling rate (A) of the water-absorbent resin in a polyethylene oxide aqueous solution at 40°C is 0.25 g g －１ ・s －１ The absorbent article according to any one of claims 1 to 4.

6. The absorbent article according to any one of claims 1 to 5, wherein the absorbent further comprises a hydrophilic fibrous material, and the mass of the water-absorbent resin is 50% by mass or more and less than 100% by mass of the total mass of the water-absorbent resin and the hydrophilic fibrous material.

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